(1) Field of the Invention
The present invention relates to an improvement of non-aqueous electrolyte secondary cells.
(2) Description of the Prior Art
Non-aqueous electrolyte secondary cells represented by lithium ion secondary cells have high energy density and high capacity, and thus are widely used as the driving power sources of mobile information terminals.
Generally, non-aqueous electrolyte secondary cells use a positive electrode made of transition metal compound oxide containing lithium, a negative electrode made of carbon material such as graphite, and a non-aqueous electrolyte containing a lithium salt dissolved in a non-aqueous solvent that is made of chain carbonic acid ester or cyclic carbonic acid ester. In such cells, although there is migration of lithium ions between the positive electrode and the negative electrode upon charge and discharge, an internal short circuit caused by dendrite lithium does not occur because there is no generation of metal lithium. Therefore, such cells excel in safety.
However, such non-aqueous electrolyte secondary cells can be problematic in that decomposition of the non-aqueous solvent caused by a reaction between the non-aqueous solvent and the electrodes upon charge and discharge results in insufficient cycle characteristics. A particular problem is that in the case of overcharge, lithium ions are released excessively from the positive electrode and stored excessively in the negative electrode. This increases the reactiveness of the electrodes to the non-aqueous solvent, resulting in deterioration of cell characteristics.
In view of these problems, when using such cells, there is employed an overcharge preventive means that utilizes a current-cutting device. However, it takes long before conventional current-cutting devices operate because they operate only when the internal pressure of the cell increases, and thus there is a doubt as to ensuring safety in the case of an intense increase in internal cell temperature.
Proposed methods to solve the above problems include techniques of adding various additives that are capable of improving safety in the non-aqueous electrolyte. The following methods are those proposed to improve cycle characteristics:
(1) Forming a coating film on the surface of an electrode plate with the use of a non-aqueous electrolyte having vinylene carbonate and ethylene sulfite (see, for example, Patent Reference 1);
(2) Forming a coating film on the surface of an electrode plate with the use of a non-aqueous electrolyte having ethylene carbonate and ethylene sulfite (see, for example, Patent Reference 3, where the ethylene sulfite is referred to as glycol sulfite); and
(3) Forming a coating film on the surface of an electrode plate with the use of a non-aqueous electrolyte having a tert-butylbenzene derivative (see, for example, Patent Reference 4).
Proposed methods to prevent overcharge include the following technique (4):
(4) Speeding up the response of the current-cutting device by adding in the non-aqueous electrolyte a phenylcycloalkane derivative provided with the ability to generate a gas by rapidly decomposing at the time of overcharge (see, for example, Patent Reference 5, where the phenylcycloalkane derivative is referred to as a cycloalkylbenzene derivative).
Patent Reference 1: Japanese Unexamined Patent Publication No. H11-121032 (pp. 2 to 3)
Patent Reference 2: Japanese Unexamined Patent Publication No. 2002-25611 (page 2)
Patent Reference 3: Japanese Unexamined Patent Publication No. H9-120837 (pp. 2 to 3)
Patent Reference 4: Japanese Unexamined Patent Publication No. 2001-167791 (pp. 2 to 3)
Patent Reference 5: Specification of Japanese Patent No. 3113652 (p.p 2 to 3)
However, the techniques (1) and (2) cannot satisfactorily inhibit the decomposition of the electrolyte solution when the cell is exposed to a high temperature (approximately 100° C.) or when charge and discharge are repeated at around the upper limit (40 to 60° C.) of normal use temperature of a cell. As a result, sufficient cycle characteristics cannot be obtained, and there is a danger of solution leakage caused by an increased internal pressure resulting from solvent decomposition, when the cell is exposed to a high temperature (approximately 100° C.). Furthermore, there is a problem of smoking or the like when the cell is heated abnormally by overcharge.
The technique of (3) is not sufficient in the effect of enhancing cycle characteristics because the coating film formed on the electrode plate is coarse, and there is a problem of smoking at the time of overcharge.
The technique of (4), although capable of preventing overcharge, allows gradual decomposition and polymerization of additives upon charge and discharge at around the upper limit (40 to 60° C.) of normal use temperature of a cell, so that the internal resistance of the cell is increased. This presents the problem of high cycle deterioration.